1. Field of the Invention
This invention relates to improvements in lead acid storage battery cells with regard to reduced water consumption and improved operating life. More specifically, it relates to the design of a simple and reliable sealed or valve regulated (VR) battery cell that has benefits not available in existing designs. The invention will be described in terms of the lead-acid variety used in stationary applications, but may be equally applied to other applications.
2. Description of Related Art
a. Traditional Flooded Cells
The traditional flooded cell as illustrated schematically in FIG. 1 has at least two electrodes formed as plates containing electrode-active materials. The plates are completely immersed in an electrolyte solution, i.e. the active area of each plate which contains the active material is completely immersed. The typical lead-acid battery cell has a sulfuric acid electrolyte solution, a positive electrode containing an active material of PbO.sub.2 and a negative electrode containing an active material of Pb. Customarily, the terms "anode" and "cathode" are employed to refer to the functions of the respective electrodes during cell discharge. A rigid but porous separator between the plates prevents any electrical contact between them. The positive and negative plates are electrically connected to respective positive and negative terminals positioned outside the cell for connection to an electric circuit.
A typical application for such a battery is standby power for computers or telephone systems. Here, the cells are placed on racks, connected in series, and charged or "floated" continuously at a controlled voltage.
This continuous charging has the effect of consuming water by electrolysis; that is, water is broken down into oxygen gas (O.sub.2) at the positive plate and hydrogen gas (H.sub.2) at the negative plate. These gasses rise to the surface of the electrolyte as bubbles and escape through a vent hole in the top of the cell.
The water consumption of flooded cells has been reduced to a minimum by charging the cells at the lowest possible voltage, but the cells still require water additions from time to time. Large stationary batteries, for example, may need watering every year in some applications which, in remote sites and with ever increasing labor costs, is expensive for the owner of the batteries.
It is, therefore, a commercial benefit to have batteries that do not require water additions, and there has been a great effort expended by the battery industry, particularly in the last decade, to develop sealed "valve regulated" (VR) cells. These VR cells use a different technology to reduce water consumption.
b. Valve Regulated Batteries
The conventional valve regulated lead acid cell (VRLA) of the conventional Absorbent Glass Mats (AGM) type, as illustrated schematically in FIG. 2, has the same kind of plates that flooded cell contains. However, instead of being submerged in a bath of electrolyte, these plates are sandwiched between absorbent sponge-like separators made from fine glass fibers. Virtually all of the electrolyte is absorbed in the sponge-like separators and there is essentially no "free" electrolyte in the cell. The VRLA cell also has a pressure relief valve, in place of the normal vent, that permits escape of pressurized gas but does not allow oxygen from the air to enter the cell.
The conventional VRLA cell has two advantages over the flooded cell. The first advantage is an immobilized electrolyte. The electrolyte being absorbed in the sponge-like separators, cannot leak out of the cell--even if the container is punctured or the cell is inverted. This advantage is very important in some battery applications (e.g. toys) but much less important in others (e.g. stationary standby power systems).
The second advantage is reduced water consumption. The aim of the VRLA design is to allow oxygen produced at the positive plate during charging to recombine with hydrogen ions and electrons on the negative plate, thereby reducing water loss. The absorbent sponge-like separator in the VR cell is deliberately designed to be less than 100% saturated, typically 95% saturated, leaving gas-permeable channels between the plates. This feature allows a process called the "oxygen cycle" (described below) to be established, which reduces the water consumption of the cell very significantly and allows the cell, at least in theory, to survive its entire design life without any water addition.
For most industrial users, this second advantage is the more important of the two because it reduces maintenance costs which, over a 20 year life of a battery, can be very substantial.
An alternative VRLA cell to the AGM cell is the gel cell. Here the electrolyte, instead of being absorbed in a glass mat separator, is mixed in the form of a gel. This gel is believed to crack in time and allow oxygen to pass through for recombination to take place in the same was an AGM cell.
c. Oxygen Cycle
FIG. 2 also shows, schematically, a valve regulated (VR) cell on charge. The oxygen gas produced on the positive plate, instead of bubbling to the surface of the electrolyte as in a flooded cell, penetrates the separator and comes into direct contact with the negative plate. This oxygen gas "depolarizes" the negative plate and recombines with the hydrogen ions generated there to form water, so that a minimum of hydrogen gas is produced. Thus the cell has a reduced water consumption.
d. Problems With VRLA Cells
VR cells of the lead-acid variety have been commercially successful and widely used in large stationary applications in the past few years. Their performance when new is usually excellent. However, as the cells age, their characteristics change for the worse and there are now many reports of field failures after only 4 or 5 years of service for cells designed for 20 years of life.
Extensive tests by my company of VR cells of different manufacturers confirm that serious variations exist in cell performance and behavior. It is the inventor's belief that the principal reason for the problems of the conventional VR cell is related to the sponge-like glass mat separators; as the cell ages, these separators become drier and drier, allowing more and more oxygen to penetrate the separators and reach the negative plate. Eventually, the rate of oxygen "transport" becomes too great and the negative plate becomes discharged.
In other words, while a small amount of oxygen transport is good and necessary for the oxygen cycle to function, too much is detrimental to the cell. The negative plate, in a sense, needs protection from "attack" by the oxygen in order to survive. However, this model of cell behavior is not yet well understood by the industry.
Berndt, Maintenance Free Batteries P.238, (John Wiley and Sons, New York 1997)discloses that an increased rate of oxygen reduction is observed in vented, flooded lead-acid batteries that have lost so much water that their connecting elements or even parts of the electrodes are no longer immersed in the electrolyte and can easily by reached by the oxygen gas. However, according to Berndt, efficient oxygen cycling to prevent the loss of electrolyte in lead-acid batteries requires immobilization of the electrolyte, and even then, the 100% efficiency attained by alkaline cells like the cell disclosed by U.S. Pat. No. 4,436,795 is not achieved. There remains a need for lead-acid batteries with improved oxygen cycling efficiency.
Accordingly, one object of the present invention is to provide a lead acid storage battery that minimizes water consumption while protecting the negative electrode from excess oxygen.
Another object of the present invention is to provide a storage battery with improved reliability.
A further object is to provide a storage battery that requires less maintenance.
Another object is to provide a storage battery that will last longer.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations pointed out in the appended claims.